Gulp.js，保存文件时监视任务运行两次

Given the following piece of code from my gulpfile.js, everytime I save or change a file, the task runs twice instead of one single time, why is that? I just want it to run one time.

var gulp = require('gulp');

gulp.task('default', function() {

  gulp.watch('server/**/*.js', function(){
    console.log('This runs twice everytime I change/save a javascript file located at server/**/*.js');
  }); 

});

我对 grunt 和名为 grunt-contrib-watch 的插件也有同样的体验.